Atopic dermatitis (AD) is the most common inflammatory disorder of the skin and is initiated by Th2-driven inflammation in response to environmental allergens. Research suggests that barrier defects enable this robust immunologic response to allergens. The skin has two barrier structures, the stratum corneum (SC) and tight junctions (TJ). It is widely accepted that the SC is dysfunctional in AD due to alterations in lipis and acquired or genetic defects in filaggrin as well as other epidermal proteins. We have recently demonstrated that epidermal TJ are also remarkably defective in AD subjects. This was largely due to reduced expression of the key TJ protein, claudin-1. Silencing claudin-1 in human keratinocytes recapitulated bioelectric and permeability defects that we observed in AD skin, and furthermore enhanced the infectivity of keratinocytes to herpes simplex virus (HSV). This latter finding suggests that TJ defects may also be responsible for AD subjects' susceptibility to cutaneous viruses such as HSV. Therefore, therapies that increase epithelial barrier integrity rather than just suppress inflammation are urgently needed. Peroxisome proliferator-activated receptor (PPAR) agonists such as Pioglitazone (Actos), play a critical role in the regulation of genes involved in epithelial proliferation, differentiation, TJ barrier and even components of the Th2 pathway. We found that Pioglitazone enhanced TJ function and increased the expression of claudin-1 and other TJ molecules in human keratinocytes. Here we propose a pilot clinical trial to determine whether Pioglitazone will repair barrier defects in AD subjects. We will use established and novel methods to visualize and quantify bidirectional barrier function both in vivo and ex vivo in well-characterized AD subjects. The results from this study will determine the effects PPAR + agonists on skin barrier function, how this translates to changes in expression of relevant SC and TJ proteins (Aim 1) and whether this will reduce the response to a known irritant (Aim 2). We are uniquely qualified to perform this study given our expertise in translational human skin barrier research, access to a large, well-characterized AD population, and experience with novel assays to measure barrier function primary human epidermal samples that we developed. This study may lead to new therapeutic strategies for both prevention and treatment of this vexing condition. Additionally, the observations made in this study will have implications for a number of other human diseases mediated in part by epithelial barrier defects including inflammatory bowel disease, celiac disease, sinusitis, food allergy and asthma.